1. Field of the Invention
The present invention relates generally to implantable defibrillator systems, and more particularly, to a method and apparatus for progressive recruitment of cardiac fibrillation using an implantable defibrillator system.
2. Background of the Invention
Implantable defibrillator systems deliver a high voltage electrical countershock to the heart in an attempt to correct or convert a detected cardiac arrhythmia or fibrillation. Due to the limitations on size and power imposed by the fact that these systems must be self-contained implantable devices, all existing implantable defibrillator systems generate an electrical countershock by charging a capacitor system to a high voltage. The electrical charge stored in the capacitor system is then delivered as a truncated capacitive discharge through two or more implanted electrodes.
To date, there have been two basic kinds of truncated capacitive discharge waveforms implemented by commercially approved implantable defibrillator systems: monophasic waveforms and biphasic waveforms. Monophasic waveforms are comprised of a single monotonically decaying electrical pulse that is typically truncated before the capacitor system is completely discharged. Biphasic waveforms, on the other hand, are comprised of a pair of decaying electrical pulses or phases that are of opposite polarity. To generate a biphasic pulse, a first pulse or phase is discharged from the capacitor system in the same manner as a monophasic waveform and then, at the point the first pulse is truncated, an H-bridge switch circuit connected to the electrodes is used to immediately reverse the discharge polarity of the capacitor system as seen by the electrodes in order to produce the second pulse or phase of the biphasic waveform that is of the opposite polarity. A typical example of the use of an H-bridge circuit to generate a biphasic waveform in an implantable defibrillator system is shown in U.S. Pat. No. 4,998,531.
In addition to monophasic and biphasic defibrillation waveforms, there have been several forms of multiple pulse defibrillation waveforms which have been proposed. The idea of using multiple pulses for defibrillation has been experimented with since as early as 1965. An external defibrillator capable of delivering multiple pulses is shown in U.S. Pat. No. 3,211,154, issued to Becker et al. Ventricular defibrillation of dogs with a multiple pulse waveform was also disclosed in 1965 by Kugelberg. Kugelberg, J., "Ventricular Defibrillation with Square-Waves", Scandinavian Society of Thoracic Surgery, Oct. 1965, pgs. 123-28. In Kugelberg's experiment, two identical spaced-apart pulses were delivered as a defibrillation waveform having a pulse length and pulse interval adjusted so that those heart cells excitable at any given moment would be defibrillated by the first pulse and refractory to the second pulse.
Since the introduction of the concept of multiple pulse defibrillation waveforms, researchers have investigated the impact of various timing relationships and discharge pathways on defibrillation effectiveness when delivering a sequence of identical pulses. Sweeney and Reid disclose that the interaction between multiple identical pulses is non-linearly related to the fibrillation cycle length, and that the spacing between multiple pulses may be a fixed percentage of the spacing between fibrillation zero crossing in the heart. Sweeney et al., "Use of Fibrillation Cycle Length to Effectively Combine Multiple Defibrillation Shocks", Supplement to Circulation, Vol. 84, No. 4, Oct. 1991, Abst. 2425; and Sweeney, et al., "Defibrillation Using a Series of Shocks Timed to the Fibrillation Cycle Length", American Heart Journal, Vol. 128, No. 3, Sept. 1994, pg. 638. Johnson et al. disclose that successive biphasic pulses delivered through two different electrodes may be either beneficial or detrimental, depending upon the delay between the two pulses. Johnson et al., "Defibrillation Efficacy for Various Delays Between Two Successive Biphasic Shocks", NASPE Abstracts: PACE, Part II, Vol. 14, No. 391, Apr. 1991, p. 715.
Different techniques for the delivery of multiple identical pulse defibrillation waveforms have been proposed and include: a sequential pulse, multiple pathway waveform as shown in U.S. Pat. No. 4,708,145 issued to Tacker, Jr. et al. and U.S. Pat. No. 5,163,427 issued to Kiemel; multiple pulses with timing based on the fibrillation cycle length as shown in U.S. Pat. No. 4,995,986 issued to Sweeney; and a low energy multiple shock waveform as shown in U.S. Pat. No. 5,107,834 issued to Ideker et al. in which two waveforms of successively lover energy are used to defibrillate.
U.S. Pat. No. 4,637,397 issued to Jones et al. shows a triphasic defibrillation system in which two of the three multiple, adjacent phases of the waveform are generated so as to be a predetermined percentage of the discharge voltage of the primary defibrillation phase. The Jones reference, however, is more accurately characterized as a single pulse defibrillation waveform in the same way that a biphasic waveform is considered a single pulse due to the fact that successive phases of the waveform are delivered sequentially with essentially no delay between phases other than any delays inherent in the switching circuitry used to alternate the polarity of each phase of the waveform. In addition, the triphasic waveform suggested by Jones et al. has been shown to be no more effective than a traditional biphasic waveform. Manz, M. et al., "Can Triphasic Shock Waveforms Improve ICD Therapy in Man?", Supplement to Circulation, Vol. 88, No. 4, Part 2, Oct. 1993, Abst. 3193.
Although several different approaches have been proposed for multiple pulse defibrillation waveforms, to date, none of these approaches has resulted in a practical defibrillation waveform which has been shown to consistently reduce defibrillation thresholds and which could be successfully implemented in an implantable defibrillator system. It is believed that the lack of an accepted theory for exactly how multiple pulse defibrillation waveforms operate to correct a fibrillating heart has impeded further development and enhancement of the multiple pulse defibrillation waveform. Accordingly, it would be desirable to provide a method and apparatus for progressive recruitment of cardiac fibrillation using an implantable defibrillator system that arises out of an improved understanding of the nature and effect of a multiple pulse defibrillation waveform on the fibrillating heart.